This invention relates to a method for testing papers of value, in particular bank notes, and to an apparatus for carrying out the method having a measuring plane, a device for translationally moving a paper of value in the measuring plane, at least one radiation source for irradiating first and second areas of the measuring plane and a detector disposed in the dark field with respect to a radiation source for detecting the radiation diffusely transmitted by a paper of value in the first irradiated area of the measuring plane.
Numerous methods and apparatuses for testing papers of value are known. The test itself can be directed to so-called authenticity features of the papers of value, on the one hand, and to the condition of the papers of value, on the other hand. In particular the latter test is applied in connection with used bank notes since they are subject to greater wear as a result of their continuous use. Depending on the nature and extent of the wear the notes are withdrawn and replaced by newly issued notes. Features used for assessing the condition of bank notes are e.g. holes, tears, missing parts, dog-ears, dirt and stains on the notes. In contrast, the notes can be tested for authenticity e.g. in terms of IR-transmitting or IR-absorbent ink prints, dimensions such as length and width, colorfastness, printed image, opacity and the like. Some apparatuses also provide for combined testing of condition and authenticity features.
GB-A-2 107 911 discloses an apparatus for testing bank notes which evaluates solely the authenticity of a note both by an optical test relating to color reflectance and IR opacity and by a length test. The note is moved along a measuring plane and scanned along three lines in order to determine IR opacity and color reflectance. Opacity measurement is done by irradiating the note with light in the infrared wave range and detecting the IR radiation transmitted through the note by means of a detector disposed xe2x80x9cin the bright field.xe2x80x9d Bright-field measurement means that the detector is reached directly by radiation from the radiation source if no note is present, and if a note is in the measuring plane it detects the radiation transmitted through the note directly from the radiation source (bright-field measurement). For measuring color reflectance a radiation in the visible wave range is additionally directed to the surface of the note, and the radiation reflected by the note surface is detected with a reflectance sensor. The detected transmission and reflectance radiations are compared with reference values in order to test the authenticity of the note. Testing of the length of the note is likewise done by means of the IR radiation source in that the leading edge of the note is detected therewith when the note is supplied to the measuring station while the end of the note is determined by a second sensor. However, there is no condition testing of the note.
DE-A-196 04 856 discloses an apparatus and method for testing optical security features with metallically reflecting layers such as holograms and the like as to exact positioning in the note, edge form (fraying of the contour) and completeness (holes, missing parts). One thus tests the condition of said security features in bank notes returning from circulation to the bank for example. The condition test of said metallic security features is done in transmitted light, similarly to the above-described opacity test. However, bright-field measurement as described above has proved unsuitable since an opposite arrangement of radiation source and detector would lead to metrologically adverse overdriving of the detector through direct incidence of radiation in the spaces between consecutive notes. Holes in the material under measurement would have the same effect. DE-A-196 04 856 accordingly proposes dark-field measurement. In dark-field measurement the detector is aligned with the radiation source so as not to receive any direct radiation from the radiation source when no note is present, but to be reached substantially only by radiation from the radiation source when a note is present, the radiation transmitted through the note being detected. Accordingly the detector is disposed with respect to the transport plane of the note so that light passing through the bank-note paper beside the metal layer or through its being damaged (holes, abrasion in the area of folds) is only measured insofar as it is scattered by the paper. However, this method cannot determine holes or other flaws in the paper but only in the metallic coating. Furthermore, dark-field measurement is unsuitable for determining a flaw in the paper itself since the detector cannot clearly ascertain e.g. in the case of a hole whether it is an especially opaque and therefore nontransparent place in the note or in fact a hole in the note since the detector disposed in the dark field would receive no radiation either way.
EP 0 537 513 A1 describes an improved authenticity tester for bank notes which is intended to recognize even especially good forgeries. The device is accordingly elaborate and it is proposed that dark-field measurements be performed both with IR radiation and with red light, on the one hand, and reflectance measurements both with respect to the reflectance of red irradiated light and with respect to the reflectance of green irradiated light, on the other hand. The quality of authenticity testing is thus increased by a plurality of independent authenticity tests being performed. No condition testing of the note is performed with this device.
DE-PS 20 37 755 discloses an apparatus for testing vouchers which reliably tests the authenticity of bank notes containing fluorescent fibers. The note is exposed on one side to radiation exciting the fluorescent substances, and the resulting fluorescent radiation emitted by the note is detected on both sides of the note. The detectors for fluorescent radiation are disposed in the dark field with respect to the excitation radiation source so that a further detector can be disposed in the bright field on the side of the note opposite the excitation radiation source. The detector disposed in the bright field is intended to recognize the condition of the paper of value by recognizing deficient paper density, splices, tears, inaccurate interfaces, faulty watermarks and lacking security threads by the opacity of the paper. However, this also involves the problem that direct incidence of light on the detector disposed in the bright field can lead to overdriving of the detector. In particular this detector arrangement does not permit reliable differentiation between relatively transparent, e.g. thin or unprinted, paper and holes.
The aforementioned apparatuses are either fully unsuitable for condition testing of papers of value because they relate only to authenticity testing, or only partly suitable because they cannot reliably determine holes, tears, missing parts, dog-ears and the like. Dark-field measurement involves the problem of the detector failing to determine a measured value both when detecting a flaw and when detecting a very opaque area so that it is impossible to differentiate between a hole and high opacity. In bright-field measurement the detection of a hole leads to overdriving of the detector tor at least to a high measured value which cannot be reliably distinguished from a likewise high value from a very weakly opaque area of the note.
For this reason one customarily determines flaws in bank notes using a separate hole detector, usually designed as an ultrasonic sensor. This additional hole detector involves additional costs which are not justifiable in every case. Thus, a bank note testing device detecting the condition of the notes and optionally easily testable authenticity features would frequently be sufficient for use in small banks, exchange bureaus, casinos and the like.
The problem of the present invention is therefore to propose a method and an apparatus for testing papers of value which permit reliable recognition of flaws in bank notes in an inexpensive way.
This problem is solved by a method and an apparatus according to the present invention.
According to the invention the opacity of a note is measured both in the bright and dark fields and the determined measured values are compared. Since neither bright-field measurement nor dark-field measurement taken alone permits a reliable statement about a flaw in the note, the inventive solution provides for comparison of the two values in order to recognize whether a flaw or a slightly opaque or highly opaque area of the note is involved. When a slightly opaque area of the note is detected, bright-field measurement states no meaningful value but dark-field measurement is clear. When a highly opaque area of the note is detected, however, dark-field measurement states no meaningful value but bright-field measurement is clear.
This principle constitutes a comparatively inexpensive solution in particular because the transmission measurement method (bright-field or dark-field) customarily used for testing the opacity of bank notes need not be equipped with an additional ultrasonic sensor as a hole detector, but instead a further transmission measurement (dark-field or bright-field) is effected so that one can omit for example a special evaluation unit for the ultrasonic sensor. Due to the duplication of several components, such a tester is much less expensive to produce as a mass-produced article.
The test result is exacter the better the resolving power, i.e. the smaller the distances between detected bank note areas and the higher the degree of overlap of the note areas measured in the bright field and those measured in the dark field. An optimum result is obtained when the note areas measured in the bright field and the note areas measured in the dark field are identical and the total note is tested in extremely small steps. The method can be considerably accelerated when adjacent note areas are measured alternately in the bright and dark fields. However, this only permits reliable detection of flaws in the bank note which are so great that they are detected both by bright-field measurement and by dark-field measurement.
This principle can be realized in different ways in terms of procedure and apparatus. Thus, one radiation source and one detector can be used for bright-field measurement and dark-field measurement in each case. However, a cost reduction can be obtained by using instead of one detector and radiation source for bright-field measurement and dark-field measurement in each case, i.e. instead of two detectors and two radiation sources, either only one common radiation source with two detectors or one common detector with two radiation sources.
Using one common radiation source with two detectors, there are two possibilities. Either the radiation source irradiates two separate areas of the measuring plane, the first detector being disposed in the dark field of one irradiated area and the second detector in the bright field of the other area, or the radiation source irradiates only one area of the measuring plane, the first detector being disposed in the dark field of said irradiated area and the second detector in the bright field thereof.
Using one common detector with two radiation sources, there are likewise two possibilities, since the two sources can irradiate either two different areas of the measuring plane or the same area of the plane, the sources being disposed in both cases so that the common detector is in the dark field with respect to the first source and in the bright field with respect to the second source. Furthermore, the embodiment with one common detector necessitates that bright-field and dark-field measurement be performed at separate times. This can be obtained by driving the radiation sources accordingly or, in case two different areas of the note are irradiated, by darkening the detector with respect to a certain area in each case, or by aligning the detector with a certain area in each case. It is most favorable procedurally to drive the first and second radiation sources separately. A special embodiment of the invention provides that at least one radiation source is designed as an IR radiation source. This permits simultaneous testing of the note for IR permeability since many notes are printed with special inks which either absorb IR radiation or, more frequently, are permeable to IR radiation.
The embodiment with two separate radiation sources furthermore offers the possibility of additional reflectance measurement since a reflectance receiver on the side of the radiation sources can be used to test the printed image of a note by the light, reflected by the note. Further advantages and properties of the inventive solution will become clear from the following description and reference to the figures.